/Engine/Plugins/FX/Niagara/Source/NiagaraEditor/Public/ViewModels/NiagaraSystemViewModel.h
Gets a multicast delegate which is called any time the array of emitter handle view models changes.
[FOnEmitterHandleViewModelsChanged](API\Plugins\NiagaraEditor\ViewModels\FNiagaraSystemViewModel\FOnEmitterHandle-) & OnEmitterHandleViewModelsChanged()